1. Field of the Invention
The present invention relates to a multilayer high-frequency circuit board.
2. Description of the Related Art
In a high-frequency circuit board used in a high frequency band of, for example, a microwave transmitter or a millimeter wave transmitter, impedance matching at an interface among a plurality of circuits in the high-frequency circuit board is important to cause the high-frequency circuit board to perform a desired performance.
Placing an intermediate board between a coaxial connector and a multilayer high-frequency circuit board is well known from Jpn. Pat. Appln. KOKAI Publication No. 2004-281120 in order to significantly reduce an impedance disturbance in an electrical connection between the coaxial connector and multilayer high-frequency circuit board.
In recent years, however, high-precision impedance matching is strongly demanded in an interlayer connection circuit by which multilayer signal lines are connected in a multilayer high-frequency circuit board.
FIG. 5 schematically shows a configuration of a conventional interlayer circuit in which impedance matching is performed. The conventional interlayer circuit includes a complete circular signal line connecting portion 3 connected to a signal line 2 in a signal line layer 1c between two ground (GND) layers la and 1b, and an approximately ring-shaped ground connecting portion 4 provided concentrically around the signal line connecting portion 3 in the signal line layer 1c. The ground connecting portion 4 has an approximate C shape which surrounds the signal line connecting portion 3 and a part of which corresponding to the signal line 2 is notched. The ground connecting portion 4 is electrically connected to the ground layers 1a and 1b by a plurality of electrical connection through-holes 4a arranged on the ground connecting portion 4 concentrically around the signal line connecting portion 3.
As shown in FIG. 6, impedance matching in this conventional interlayer circuit is performed by selecting values of three parameters which include a distance A from a center O of the signal line connecting portion 3 to an outer edge of the signal line connecting portion 3; a distance B from the center O to an outer edge of the ground connecting portion 4; and a distance D from the outer edge of the signal line connecting portion 3 to an inner edge of the ground connecting portion 4.
However, since impedance matching is performed in such a conventional interlayer circuit on a basis of these three parameters, there is a limit to achievable precision in a tuning of impedance matching.